The invention has been developed primarily for carrying alternating current and will be described hereinafter with reference to that application. It will be appreciated however that the invention is not limited to that particular field of use and is also suitable for carrying DC current.
It is known to produce a superconducting wire by filling a tube of silver or silver alloy with particulate high-temperature ceramic superconductor. By way of example, it is known to use a superconductor selected from the class known as “BSCCO” superconductors. Of these, the subgroup known as BSCCO-2223 is most preferred.
Once the superconductor is placed in the tube further process steps occur. More particularly, the tube is rolled to reduce its cross-section to compact it. Further rolling is carried out to eventually flatten the tube which is heat treated at least once. This processing assists in aligning the plate-like grains of the superconductor.
It is also known to combine a plurality of the wires, as referred to above, to form a superconducting tape. Each of the wires is referred to as a “filament” and the resultant tape as a multifilament tape. The filaments are initially reduced in cross section and placed within a common tube of silver or silver alloy. This common tube is then “textured”, in that it is reduced in cross section and then flattened. This generally results in the filaments of particulate superconductor each having a thickness below about 40 micrometres. As the limits to the practicable width of the filament are not much more than 10 times its thickness, multiple filaments are needed to provide a substantial supercurrent capacity. In some cases the assembly is twisted to “transpose” the filaments with a view to equalising the supercurrent carried by individual filaments.
It has become accepted that the flexibility or other physical requirements limit the thickness of the finished tape to about 0.3 mm. Consequently, large numbers of tapes are needed to make superconducting power cables with adequate supercurrent capacity. It is known to use two and often more layers on a tubular support. However, the more tapes there are:                the more difficult it becomes to substantially equalise the distribution of supercurrent between the tapes; and        the more the performance of each of the tapes is degraded by the magnetic field and other effects of the remaining tapes. This degradation is particularly applicable to AC losses.        
It is an object of the present invention, at least in the preferred embodiment, to overcome or substantially ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.